This invention relates to integrated circuit fabrication processes, and more particularly to a method of providing complementary contactless vertical bipolar transistors.
Recent developments in contactless bipolar transistor technology make possible smaller bipolar devices with simplified interconnect structures, thereby reducing power consumption and increasing yield. These developments include the use of polysilicon for the emitter structure, which permits the fabrication of extremely shallow emitters; the use of composed masking, in which critical spacings between the base and emitter are defined in the polysilicon at the same mask level; the use of polysilicide instead of the conventional copper-doped aluminum for emitter interconnect, which provides thin reliable current carriers; and the use of a high degree of self-alignment, by which, for example, the polysilicide covering the base, emitter and collector regions is aligned to the polysilicon features, and the n+ polysilicon is aligned to the emitter.
The developments recited above are described in U.S. Pat. No. 4,609,568, issued Sept. 2, 1986 to Koh et al., which teaches a process for fabricating self-aligned regions of metal silicide on bipolar integrated circuits having self-aligned polysilicon emitter and base contacts. Polysilicon is deposited on the silicon substrate, then suitably doped and coated with a layer of protective nitride. After a process sequence involving an oxidation mask (also known in the art as a silicide exclusion mask) and a poly definition mask, base, emitter and collector contacts are formed in the polysilicon, and polysilicon areas from which silicide is to be excluded are defined. Subsequently, (a) a thermal drive-in step forms a base and emitter self-aligned to their respective polysilicon contacts; and (b) a thermal oxidation step passivates the active areas and form an oxide over areas from which silicide is to be excluded, whereby a self-aligned silicide can be formed over all exposed poly.
Unfortunately, the need remains for a bipolar process that provides either vertical NPN transistors, vertical PNP transistors, or both while retaining the advantages of a poly emitter, composed poly definition mask, silicide interconnect system, and self-aligned features, all without involving an excessive number of masking steps.